Thanks for your explanation Albert, it makes things clearer.
So StablePtrs are just useful so that C code can:
1) call back into Haskell (through a foreign exported function like
doSomethingWithTheObjectIGaveYou
:: StablePtr MyObjectType -> Stuff -> IO ())
2) store them to return them later to Haskell when prompted (through a
foreign imported function like getObject :: Stuff -> IO (StablePtr
MyObjectType))
That's it?
But then,
In use case 1), how can a Haskell function modify the data addressed?
If StablePtrs cannot have their pointed value modified (either C or
Haskell-side), that mostly limits their interest, doesn't it?
2012/2/12 Albert Y. C. Lai
On 12-02-12 09:18 AM, Yves Parès wrote:
According to the documentation (http://hackage.haskell.org/**packages/archive/base/4.5.0.0/** doc/html/Foreign-StablePtr.**htmlhttp://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Foreign-St... ), StablePtrs aims at being opaque on C-side.
The doc multiply warns again and again that StablePtr, as well as whatever Ptr you get from castStablePtrToPtr, are opague (meaningless) to the C side. This is sanctioned by Haskell 2010, and GHC certainly exploits it to the fullest. The following example shows what kind of "pointer" values the C side receives for real (I deliberately do not free anything to show you more possible values):
#include
void expose(void *p, void *q) { printf("%p %p\n", p, q); } import Foreign.StablePtr import Foreign.Ptr main = do printout (0 :: Int) printout (let x = not x in x) printout ([] :: [Integer]) printout :: a -> IO () printout thunk = do p <- newStablePtr thunk expose p (castStablePtrToPtr p) -- I deliberately do not free foreign import ccall expose :: StablePtr a -> Ptr b -> IO ()
Typically the output is like 0xf 0xf 0x10 0x10 0x11 0x11 Looks more like keys of a lookup table than pointers.
I do not know what good is castStablePtrToPtr for, given that StablePtr is already translated to C side void*, so that no intermediate Ptr step is necessary. Perhaps there is a story from a historical perspective.
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